The present invention is based on the consideration that the main factors in effecting the development of a chemical reaction are:
A. The manner and intensity by which the reactants come into contact with one another;
B. The degree of agitation and thus dispersion of the reactants coming into mutual contact;
C. The shape of the space in which the reaction occurs;
D. The temperature and pressure under which the reaction or reactions take place;
E. The uniform treatment of the materials to be processed.
The apparatus according to this invention is capable of efficiently controlling all of these factors while continuously processing the reacting materials.
At present, there is a growing trend toward the use of continuous-flow chemical reactors as replacements for batch-type reactors employed heretofore. The continuous-flow reactors are readily adapted to be used in conjunction with on-stream chemical composition analyzers and other on-line monitoring equipment which continuously control the processing as it takes place, thereby achieving greater efficiency and cost savings in the production of various chemical products. For example, such reactors find particular applicability in the polymerization of resins and in other chemical reactions where the process requires that reactants have predictable residence times, in order to monitor the completeness of the reaction. Residence time, i.e., the amount of time spent by an identifiable group of reactants in a defined location within a reactor, is an extremely important factor in maximizing the output and minimizing time and costs. This is generally true in any chemical syntheses where significant residence time is required in order to satisfy reaction kinetics. Generally, organic reactions, and especially polymerizations, require significant residence times during mechanical processing. This is especially true in large scale, continuous operations. Being able to accurately predict and control residence time has been considered a primary goal in the continuous processing of chemical reactants, especially viscous reactants such as those encountered in polymerization reactions.
Also, the success of commercial chemical reactions, such as polymerization, is measured by the properties, and more specifically, the uniformity of properties, exhibited by the resulting product and by the quantity of product or yield produced from a given amount of starting materials. In many cases, in order to achieve such uniform properties, one must be able to control the temperature to which the process materials are subjected and the homogeneous manner in which they are mixed. These two factors, mixing and temperature control, are interrelated in the processing of viscous materials. Such materials, because of their low thermal conductivity, are normally very poor heat transfer media. However, it is possible to improve the heat transfer in such materials by effecting continuous agitation thereby keeping the body of the reaction mixture continuously in motion and well intermixed. This becomes more difficult in viscous masses such as those formed by high polymers. Ideally, in designing an apparatus to process such materials, one should provide a system which is capable of efficient heat transfer while effecting a homogeneous intermixture of reactants with a minimum of energy expenditure. All three factors should be provided for; otherwise the process will become uneconomic or the products will not possess the uniform properties required of commercial materials. A continuous flow reactor would be an ideal way to processs viscous materials; however, presently available reactors are not readily adapted to this use.
Typical continuous-flow reactors are described in an article by Arthur P. Weber entitled "The Design of Commercial Continuous Reactor Systems" in the January 1953 issue of Chemical Engineering Progress magazine and in U.S. Pat. No. 3,752,653. One type of continuous-flow reactor described by this patent is a closed-vessel having an inlet for the introduction of process materials at one end of the reactor, one or more central draft tubes having a series of inlet apertures at one end and an exhaust outlet at its other end, and an outlet for the discharge of processed materials at the end opposite to the inlet. Process material is flowed through the draft tube by means of impeller blades spaced apart axially within the cylindrical passage formed by the draft tube. The blades recirculate the process material within the vessel and through the draft tube while the material is being reacted. The presence of the draft tube in the vessel makes possible a computation of the residence time of the material in the vessel, and hence, enables one to control and measure the completeness of reaction. Although this reactor operates satisfactorily on low to moderate viscosity materials, a continuous-flow reactor which has an improved agitating and control capabilities adapted to the processing higher viscosity materials is highly desirable. A typical high viscosity material, as contemplated herein, would have a viscosity of about 100,000 centipoises or above. The above-described system is not effective to process materials which are highly viscous as they are introduced into the reactor or materials which become highly viscous during the course of processing. Obviously, this is a serious limitation in commercially important processes such as polymerizations, which in many cases involve highly viscous reactants or products.
Another type of reactor which can process high viscosity materials is the extruder-type, such as described in U.S. Pat. No. 3,765,481. Although reactors of this type are able to provide efficient mixing and transfer, such apparatus cannot provide a means for accurately controlling the residence time, and hence, the homogeneous treatment of the material being processed.
The present system can be used either alone or in conjunction with other systems to overcome the disabilities found in conventional systems, such as the ones described above, and at the same time provide the advantages of being able to continuously and accurately control critical processing variables such as feed rate, temperature, rate of shear, residence time, etc., for optimum processing.
Another reactor adapted for polymerization reactions is described in U.S. Pat. No. 3,567,402. This patent describes an adiabatic batch polymerization reactor having a vertical jacketed tube axially positioned within the reactor for the purpose of controlling the temperature in a restricted reaction zone. This arrangement is satisfactory for single batch polymerization reactions; however, unlike the present invention, this arrangement is not suitable for continuous processing. Nor is this type of reactor adapted to the non-adiabatic processes. Because of this and other limitations inherent in its design, this reactor is restricted in the variety of materials and types of reactions that it can process effectively. An additional advantage which the reactor of the present invention has over this type of reactor is that, because of its unique configuration and design, it does not require a control valve to control the flow of materials within a jacketed tube nor does it require temperature sensing means in the tube in order to monitor and control the reaction process.
With this in mind, it is the object of the present invention to overcome the foregoing disadvantages and to provide an improved continuous-flow reactor of one or more stages for processing high viscosity materials.
It is another object of the present invention to provide an apparatus for carrying out physical or mechanical mixing and chemical reactions more efficiently and to provide a simple and accurate method for predicting and controlling the continuous mixing and advancement of process materials, thus enabling one to determine accurate residence times for the materials and more carefully control mixing and reaction conditions.
As a further object, the present invention provides a continuous flow reactor having means for uniformly applying the high mechanical and shear forces necessary to process viscous materials flowing through the reactor, thereby improving the reactor performance and quality of the product.
It is another object of this invention to provide a well defined flow path for process material resident in each stage of the reactor thereby assuring the homogeneous treatment of material while advancing a selected portion of the material at a uniform controllable rate.
It is still another object of this invention to employ agitator and mixing elements having a simple mechanical arrangement to process high viscosity materials with greater efficiency.
More specifically, the present invention provides a means by which a reactor comprising a hollow cylindrical vessel having mounted within it one or more stage barriers which, in conjunction with the vessel wall, define the boundaries of each reaction stage within the vessel. This stage barrier uniformly restricts the flow of material within each stage of the reactor and is designed to provide a passage for uniformly controlling the egress of process materials from each stage. The vessel has inlet means for the continuous introduction of process material at one end of the stage and an outlet means for the discharge of process materials at the other. A mixing assembly is mounted within each stage for uniformly mixing and recirculating process materials in each stage at a predetermined rate. Each mixing assembly includes a central helical screw, one or more helical ribbon agitators, and a draft tube all of which are attached to a central rotor shaft. The rotor shaft extends longitudinally through the vessel along its central axis and passes through the central portion of the stage barrier. The draft tube is mounted coaxially within each stage thereby defining an annular passage between the stage wall and the tube. The draft tube controls the direction of flow of process materials within each stage and assures that the material being processed is uniformly advanced within the reactor at a fixed, preselected rate. The helical screw and the helical ribbon agitator are coaxially mounted to the rotor shaft, with the screw occupying the cylindrical passage within the draft tube. The ribbon agitator has a pitch opposite to that of the screw and occupies the annular passage between the draft tube and the reactor wall. When rotated, the helical ribbon agitator and the helical screw cooperate with the draft tube, the stage barrier and the reactor walls to establish a uniform recirculation of process materials in a fixed path within the stage. The direction of recirculation can be varied depending on the configuration of the reactor and process requirements as long as uniformity of treatment is maintained. The elements of the mixing assembly, in conjunction with the pressure created by materials being continuously introduced into the reactor, also cooperate to advance a preselected fixed portion of the process material through the passage formed by the stage barrier. The internal pressure produced by the net flow of the process material, out of each stage and the stage barrier, cooperate to prevent material from backmixing, or flowing back into the stage from which the material is exiting. This assures uniform treatment of the process material.
The reactor may have one stage or multiple stages, depending upon the degree of control required in processing the material. As the number of stages is increased, the average residence time required to complete the reaction decreases and the possibility of non-homogeneous treatment of materials is reduced.
If desired, heat exchange means may be provided to supply or extract heat in one or more of the stages, depending upon the preferred operating temperatures required for given processes and materials. Heat exchange means can be jacketed housings surrounding the reactor shall or the mixing elements may be hollowed out to provide enclosed passages for carrying a heat exchange fluid within the reactor elements. Also, temperature sensing means, such as thermocouples, may be provided in order to monitor the temperatures of the process material.
The reactor may be operated in any orientation. When operated in the horizontal position, the helical ribbon provides the additional advantage of being able to enhance the separation of vapor from the viscous process material.